I switched to oreilly's premium transhydraulic oil on all 3 of my keeper trucks, and they instantly started shifting smoother. It is 10 weight, and C4 rated. I unhooked my cooler hoses, removed the trans pan, and blew compressed air thru the cooler lines. Close enough to a full flush for me.
You can run any oil in the mt654cr as long as it is Allison C4 rated. You can even use TES-295 full synthetic if your wallet can stand it.
15-40 wt. engine oil is to be used off road, in 120 degree F operating conditions, loaded at slow speeds. Think desert conditions with no hard paved roads, Maximum rolling resistance. The reason allison lists the 15-40wt. for this application is that engine oil has the highest boiling point of all the approved fluids listed.
For those of you who insist on running 15-40wt. engine oil in cold climates, let your truck warm up for 30 minutes, or install a trans pan electric heater. If you fail to let the transmission warm up, you will be rewarded with a shorter transmission life.
I do have one truck currently that is giving shifting issues, it needs the modulator cable removed and cleaned. A small adjustment on the injection pump end of the modulator cable makes a big difference in shift quality. Make sure you have 2 springs on the pump end of the cable, and that the linkage is sliding/returning smoothly throughout the throttle travel.